So first, open the Calendar program and click on the calendar you’d like to get rid of from the left-hand list.
If you don’t see this, click the “Calendars” button on the toolbar.
Once you’ve got the correct one selected, go up to the menus and choose File> Export> Export.
Your Mac will ask you where you’d like to save your calendar file, so put it somewhere handy. Then we’re going to reimport it to a different calendar—do that by choosing File> Import and picking the .ics file you saved out a second ago.
The application will then want to know which of your calendars you’d like to add those existing events to. Make sure you pick, say, one of your current iCloud calendars (or at least one that you know you want to keep).
Click “OK,” and all of those events will be brought into that calendar!
One thing I like to do before I remove any unwanted calendars is make certain that the events imported properly. You can do that by deselecting every other calendar in your sidebar so that you only see the calendars you just imported from and to:
And here’s a bonus tip for you guys (hooray!): If you hold down Command and click any of the checked boxes next to your calendars, that will deselect all of them. That’s really useful for when you want to see the events on only a calendar or two. (If you’d like to reselect all of them at once when you’re finished, just Command-click one of the empty boxes. Easy-peasy!)
Anyhow, while you’ve got the calendars in question selected and all of your other ones hidden, you can easily compare events and feel confident that there’s nothing on the calendar that you’re about to remove that didn’t get duplicated on the destination calendar. Make sense? All right, you’re ready to start deleting! To do so, just right- or Control-click on the calendar you want to remove and pick “Delete.”
Of course, you’ll get a warning, but that’s no big surprise. Hey, thanks for trying to help, computer.
Once you click “Delete” on the warning pop-up, you’re all finished. Repeat these steps for any subsequent calendars that you’d like to remove, and your program will be all neat and clean and stuff. Hooray for neat and clean…and, um, stuff!